Influence of Annealing on the Grain Growth and Thermal Diffusivity of Nanostructured YSZ Thermal Barrier Coating

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1 J. Mater. Sci. Technol., Vol.22 No.6, Influence of Annealing on the Grain Growth and Thermal Diffusivity of Nanostructured YSZ Thermal Barrier Coating Na WANG, Chungen ZHOU, Shengkai GONG and Huibin XU Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing , China [Manuscript received October 13, 2005, in revised form February 27, 2006] The nanostructured zirconia coatings were deposited by atmospherically plasma spraying. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction were used to investigate the microstructure of the zirconia coatings. Thermal diffusivity values at normal temperatures have been evaluated by laser flash technique. Effect of annealing on the microstructure evolution of the zirconia coating has been performed. The grains and thermal diffusivity are increased with increasing annealing time and temperature. The grain growth is according to the GRIGC (the grain rotation induced grain coalescence) mechanism. The increase in thermal diffusivity is attributed to the grain growth and the decrease in porosity of nanostructured zirconia coatings. KEY WORDS: Grain size; Thermal diffusivity; Thermal barrier coating; Plasma spraying 1. Introduction Thermal barrier coatings (TBCs) are widely used since the 1970s to insulate metallic components in gas turbine and diesel engines from high temperatures. These coatings can increase the gas inlet temperature without raising the base metal temperature [1,2]. The duplex system ZrO 2 /Y 2 O 3 and MCrAlY protects components of industrial gas turbines (e.g. blades and vanes) and aero engines against high temperature oxidation and high temperature corrosion. A thermally grown oxide (TGO) would form between the ceramic topcoat and the bond coat due to high temperature oxidation of the bond coat [3 5]. It was reported that the yttria stabilized nanostructured zirconia coatings with lower thermal conductivity and higher coefficient of thermal expansion have a higher degree of thermal insulation and thermal cycling life [6,7]. Its perfect properties are helpful to increase the component durability of the gas turbine engine [8,9]. Thermal barrier coatings are used at high temperatures. Grains would grow at high temperature. However, few reports on the grain size and thermal diffusivity after high temperature heat treatment have been published since the thermal diffusivity of the coating is tightly related to grain size. The purpose of this paper is to investigate the influence of annealing on the grain growth and thermal diffusivity of nanostructured YSZ thermal barrier coating. 2. Experimental The method of APS was sprayed using the material of 8 wt pct Y 2 O 3 partially stabilized nanostructured zirconia powder on the columnar stainless steel. The stainless steel is 10 cm in diameter. Then the sample after spraying was put into cool water immediately so that the coating would break away from the stainless steel sted, and has a shape of 10 cm in diameter. The coatings were grinded to 1 cm in thickness. Ph.D., to whom correspondence should be addressed, cgzhou@buaa.edu.cn. And the parameters for plasma spraying were given in our previous investigation [10]. The heat-treatment tests were performed for 15 h at 900 C, 1000 C, 1100 C, 1150 C and at 1100 C for 15 h, 70 h, 100 h, 300 h, respectively. The dependence of grain size of zirconia coating on the heat-treating temperature was also explored using a D/max 2200PC X-ray diffractometer (Rigaku, Tokyo, Japan). The mean grain size of coatings were calculated using the Scherrer equation [11,12] B p (2θ) = 0.9λ/Dcosθ where D is the average dimension of crystallite, B p (2θ) is the broadening of the diffraction line measured half maximum intensity, λ (0.154 nm) and θ denote the wavelength of the X-ray and the Bragg diffraction angle, respectively. The microstructures of the starting powders and the nanostructured zirconia coatings were examined with an S-3500 scanning electron microscope (Hitachi, Tokyo, Japan) and an H-800 transmission electron microscopy (Hitachi, Tokyo, Japan). The laser-flash diffusivity method was used to examine the thermal diffusivity of the nanostructured zirconia coatings. 3. Results 3.1 Microstructure analysis Figure 1 presents the typical SEM microstructures of the surface and fracture surface of the assprayed nanostructured zirconia coating, respectively. It can be seen that the structure is very loose and there are some micro-pores in the coating. The microstructure of the coating consists of molten phase and non-molten phase, which could be seen in Fig.1. The molten particles merge together and rapidly directional solidify along the direction of thermal conduction and then form columnar particles. The microstructure is alternate by molten phase and nonmolten phase, which could be helpful to the consolidation of the topcoat.

2 794 J. Mater. Sci. Technol., Vol.22 No.6, 2006 Fig.1 SEM microstructure of the as-sprayed zirconia coating: (a) surface of the zirconia coating, (b) fracture surface of the zirconia coating Fig.2 Transmission electron microscopy image of the primitive nano YSZ powder (a) and the as-sprayed coating (b) Fig.3 XRD patterns of the as-sprayed coating and the coatings after heat-treatment: (a) for 15 h at different temperatures and (b) at 1100 C for different times The grains of the primitive powders and the assprayed coatings have been observed, as shown in Fig.2. It reveals that the grains of the coating are in the range of nm, larger than that in primitive powders, which demonstrates that the grains grow up during plasma spraying, since the temperature is high up to 3000 C. 3.2 Effect of heat treatment on grain growth Annealing has a great influence on the sizes of the grains. Heat-treatment experiments have been done to investigate the relationship between annealing and grain growth. Figure 3 gives the XRD patterns of the topcoat before and after heat-treatment at different temperatures for 15 h and at 1100 C for different times. It can be seen from Fig.3 that the coatings are consisted of tetragonal phase. The Scherrer equation was used to calculate the mean grain sizes from the result of XRD, as shown in Fig.4. It can be seen that with the rise of annealing temperature, the grains grow from 57 nm to 87 nm. Figure 5 is the TEM image of the coating after heattreatment at 1150 C, from which the growth of the grains could be seen compared to the as-sprayed

3 J. Mater. Sci. Technol., Vol.22 No.6, Fig.4 Grain size of the topcoat after heat-treatment: (a) for 15 h at different temperatures and (b) at 1100 C for different hours Fig.5 TEM image of the nanostructured YSZ coating after heat-treatment at 1150 C for 15 h Fig.7 Thermal diffusivity of the topcoat after heattreatment: (a) for 15 h at different temperatures and (b) at 1100 C for different hours ter heat-treatment at 1100 C for 300 h is good accordance with the result of TEM (Fig.6). Fig.6 TEM image of the nanostructured YSZ coating after heat-treatment at 1100 C for 300 h image. Meanwhile, for annealing time, it has the similar regularity that with the increase of annealing time, the grain sizes also increase. The grain size of the topcoat after heat-treatment at 1100 C for 300 h is 177 nm. The calculated grain size of the topcoat af- 3.3 Effect of heat treatment on thermal diffusivity Figure 7 gives the values of thermal diffusivity of the coatings after heat treatment. It can be seen that with the rise of annealing temperature from room temperature to 1150 C, the thermal diffusivity increased from mm 2 /s to mm 2 /s. For increasing annealing time, the range of thermal diffusivity is mm 2 /s mm 2 /s. the longer the heat-treatment time; the larger the thermal diffusivity. Compared with the effect of temperature, annealing time has more significant influence on the values of grain size and thermal diffusivity. Figure 8 gives the change of thermal diffusivity with the increasing of grain size of the nanostrucutured zirconia. The larger the grain size, the larger

4 796 J. Mater. Sci. Technol., Vol.22 No.6, 2006 Fig.8 Relationship between thermal diffusivity and grain size the thermal diffusivity. The dramatic increase of thermal diffusivity of nanostructure zirconia coating after the heat-treatment is clearly related to its microstructural characteristics. 4. Discussion In the present investigation, the thermal diffusivity of the plasma-sprayed nanostructured zirconia coating increased after heat-treatment at different temperatures for 15 h and at 1100 C for different times. The increase in the thermal diffusivity is attributed to the micro-pores and the increased grain size of nanostructured zirconia coatings. There exist the micro-pores between the micro powders from Fig.1, which have the similar size with the powders. The micro-pores may have an effect on phonon scattering, resulting in the decrease in thermal diffusivity [8]. For ceramic materials, the porosity would decrease after sintering at high temperatures [13,14]. Another reason of the increase in thermal diffusivity may result from grain growth. The grain growth mechanism is considered to the GRIGC mechanismthe grain rotation induced grain coalescence [15,16]. According to this model, a neck would be formed among the neighboring grains. The activation of the course is supposed to be very low [17]. So after the neck generates, the two grains would grow along the mutual direction. This results in a coherent grain-grain interface, which leads to the coalescence of neighboring grains via the elimination of common grain boundaries. The grain growth can affect the thermal diffusivity through the grain boundary interfacial thermal resistance [18]. To explain this, it is then possible to make the following estimate of the grain boundary interfacial thermal resistance. The interfaces are perpendicular to a one-dimensional heat flow path through the sample between the two faces, which are considered to present thermal resistance [19]. We labeled the samples after heat-treatment for 15 h at 900 C, 1100 C as sample 1, 2. Meanwhile, the samples after heat-treatment at 1100 C for 70 h, 300 h were labeled as sample 3, 4, respectively. The sizes of the samples are 1 mm thick and 78.5 mm 2 in surface. The sample 1 has a thermal diffusivity of mm 2 /s with an average grain size of 73 nm, which includes 6.06e8 grain-grain boundaries. The thermal diffusivity of sample 2 is mm 2 /s, and its grain size is 81 nm with 4.43e8 interfaces. At the same time, the thermal diffusivity of sample 3, 4 is mm 2 /s and mm 2 /s, and there are about 6.45e7 and 4.25e7 interfaces, respectively. The result indicates that the number of grain boundaries is decreased with increasing annealing temperatures and times. In ceramics, the thermal diffusivity is dominantly decided by phonon conduction. For thermal conductivity, Debye used an analogy of the kinetic theory of gases to derive an expression of the thermal conductivity [20] : k = C v V m Λ/3 (1) where k is the thermal conductivity, C v is the specific heat, V m is the speed of sound and Λ is the phonon mean-free path. The thermal diffusivity is proportional to its thermal conductivity coefficient and inversely proportional to thermal capacity per unit volume C v [19] : α = k/c v (2) here, α is the thermal diffusivity. By appealing to the idea that lattice heat conductivity is related to the harmony of a crystal and using dimensional arguments, Dugdale and McDonald proposed that the phonon mean free path must be of the form [20] : Λ = a/(ωγ T ) (3) where a is an inter-atomic distance, Ω is the thermal expansion coefficient, and γ is the Gruneisen parameter. Combined with the Eqs.(1) and (2); the thermal diffusivity can be expressed by the following equation: α = V m Λ/3 (4) If V m is assumed to be independent on temperature, then the thermal diffusivity is principally controlled by the mean free path. Due to the increase in grain size, the number of grain boundaries is decreased as mentioned above. The grain boundary contribution to phonon scattering is decreased and thus the mean free path for phonon scattering is increased [8]. So the thermal diffusivity of the coating is increased after heat treatment at high temperatures. 5. Conclusion The nanostructured zirconia coatings have been investigated. The grains grow gradually with the increasing of annealing time and temperature. The grain growth is considered to the grain rotation induced grain coalescence (GRIGC) mechanism. The range of thermal diffusivity is mm 2 /s and mm 2 /s with the increasing of annealing temperature and time, respectively. The increase in thermal diffusivity is attributed to the grain growth of nanostructured zirconia coatings. Acknowledgements This work was supported by the program for New Century Excellent Talents in University (NCET) and the National Natural Science Foundation of China under the contact

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